The primary purpose of this Phase I study is to determine the dose of the investigational drug O6-benzylguanine (O6-BG) that will sufficiently lower the level of O6 -alklyguanine DNA alykyltransferase (AGT), a DNA repair protein, in brain tumor tissue. This is a dose escalating study which will also evaluate the nature and frequency of therapy-related toxicities. However, the role of AGT inhibitors in modulating BCNU activity in clinical trials will be limited by the magnitude of demonstrable AGT activity in patients'tumors as well as the toxicity to normal tissue produced by inhibition of repair activity. A specific aim is to evaluate the qualitative and quantitative toxicities of O6 -BG in this patient population.